1. Field of the Invention
The present invention relates to a communication system. More particularly, the present invention relates to a method and apparatus for controlling a sleep mode operation of a mobile station.
2. Description of the Related Art
Communication systems have evolved to offer users high-speed services for transmitting/receiving a large amount of data. These communication systems, for example, include an Institute of Electrical and Electronics Engineers (IEEE) standard 802.16e communication system. A normal mode in the IEEE standard 802.16e communication system refers to a state in which the communication between a Mobile Station (MS) and a Base Station (BS) is always maintained.
In the IEEE standard 802.16e communication system, an MS monitors a DownLink (DL) to determine whether data is received from a BS. The MS monitors the DL even when the BS has no data to transmit to the MS, which consumes its power unnecessarily.
Since the IEEE standard 802.16e communication system considers MS's mobility, the MS's power consumption serves as an important factor in an overall system performance and affects the MS's mobility as well. To minimize the MS's power consumption, a sleep mode and an awake mode corresponding to the sleep mode have been provided between an MS and a BS.
FIG. 1 is a diagram illustrating a conventional method for performing a sleep mode operation in a communication system.
Referring to FIG. 1, an MS 100 remaining in an awake mode transmits a MOBile SLeeP REQuest (MOB_SLP-REQ) message to a BS 110 to transition to a sleep mode in step 101. Upon receiving the MOB_SLP-REQ message, the BS 110 determines whether to approve the mode transition to the sleep mode of the MS 100, considering situations of the BS 110 and the MS 100, and transmits a MOBile SLeeP ReSPonse (MOB_SLP-RSP) message to the MS 100 based on the determination result in step 103. The MOB_SLP-RSP message includes a listening window parameter indicating a listening window. In the listening window of the sleep mode, if the BS 110 has data to transmit to the MS 100, the BS 110 transmits a MOBile TRaFfic INDication (MOB_TRF-IND) message including an Identifier (ID) of the MS 100 to the MS 100 during the listening window.
Instead of responding to the sleep mode request (i.e., request for transition to or release of the sleep mode) of the MS 100, the BS 110 may first transmit a sleep mode request to the MS 100 in an unsolicited manner. That is, the BS 110 requests the MS 100 to enter or exit the sleep mode by first transmitting the MOB_SLP-RSP message without receiving the MOB_SLP-REQ message of the MS 100.
Upon receiving the MOB_SLP-RSP message from the BS 110, the MS 100 starts a sleep mode operation according to the MOB_SLP-RSP message. The MS 100 performs the sleep mode operation according to the listening window parameter included in the MOB_SLP-RSP message. When the MS 100 has data to transmit to the BS 110 while operating in the sleep mode, the MS 100 may transition from the sleep mode to the awake mode immediately.
If the BS 110 has no data to transmit to the MS 100, the BS 110 transmits a MOB_TRF-IND message with no ID of the MS 100 in the listening window of the sleep mode in step 105. The MOB_TRF-IND message includes a negative indication for the MS 100 since it does not correspond to the MS 100. The MS 100 decodes the MOB_TRF-IND message and then continuously maintains the sleep mode, when the ID of the MS 100 is not included.
If the BS 110 has data to transmit to the MS 100, i.e., if a Protocol Data Unit (PDU) for the MS 100 has been provided from the network, the BS 110 transmits a MOB_TRF-IND message with an ID of the MS 100 in step 107. Since the MOB_TRF-IND message corresponds to the MS 100, it becomes a positive indication for the MS 100. The MS 100 decodes the MOB_TRF-IND message and transitions to the awake mode and receives the data from the BS 110, determining that the ID of the MS 100 is included.
After the data transmission/reception between the MS 100 and the BS 110 is completed, the MS 100 and the BS 110 exchange additional MOB_SLP-REQ message and MOB_SLP-RSP message to make a transition back to the sleep mode, causing transmission of unnecessary messages, waste of UpLink (UL) and DL resources, and power consumption. In addition, the MS 100 should perform bandwidth ranging by transmitting to the BS 110 a BandWidth REQuest (BW-REQ) message in order to be allocated a bandwidth for transmitting an additional MOB_SLP-REQ message, which delays the time for which the MS 100 makes a transition to the sleep mode.
The MOB_SLP-REQ message includes a Power_Saving_Class_Type field and a Traffic_Triggered_Wakening_Flag (TTWF) field. The Power_Saving_Class_Type field is used to define one of the types described below.
1) Type 1 indicates a class corresponding to a conventional sleep mode operation. In this class type, an MS transitions to the awake mode, when data transmission/reception occurs in the listening window or when it receives a MOB_TRF-IND message with a positive indication.
2) Type 2 has a fixed sleep window. In this class type, an MS performs data transmission/reception in a listening window, and also performs data transmission/reception in a next scheduled listening window after the fixed sleep window.
3) Type 3 denotes, compared with Type 1 and Type 2 in which an MS continuously maintains the sleep mode unless it receives a mode transition request message, a class of automatically exiting the sleep mode after one sleep mode operation, i.e., after one sleep window. Type 3 is used for management messages or multicast traffic.
The TTWF field is applied only to the Power Saving Class Type 1. Specifically, the TTWF is used when an MS intends to maintain the sleep mode even though data is generated in the listening window.
For example, when TTWF=0, an MS transmits/receives data during the listening window and then transitions back to the sleep mode at the end of the listening window, i.e., at the beginning of the sleep window. The MS exits the sleep mode and transitions to the awake mode, (i) when the BS intends to transmit a Medium Access Control (MAC) Service Data Unit (SDU) for a power saving class during the listening window, (ii) when the MS transmits a BW-REQ message for a connection for the power saving class, or (iii) when the MS receives a MOB_TRF-IND message with a positive indication, i.e., an ID of the MS, from the BS. In other cases, the MS may exit the sleep mode through transaction of the MOB_SLP-REQ message and the MOB_SLP-RSP message.
On the other hand, when TTWF=1, the MS exits the sleep mode and transitions to the awake mode when the MS receives a Packet Data Unit (PDU) from the BS during the listening window or when the MS receives a management message indicating the exit of the sleep mode, e.g., a MOB_SLP-RSP message or a DL Sleep Control Extended Subheader. Also, the MS exits the sleep mode and transitions to the awake mode, even when data to be transmitted is generated in the MS or when the MS transmits to the BS a management message indicating the exit of the sleep mode, i.e., a MOB_SLP-REQ message, a BW-REQ message or a UL Sleep Control Header. In other words, when TTWF=1, the MS transitions to the awake mode when traffic data or a relevant management message is generated during the listening window.
As described above, in a sleep mode operation of the IEEE standard 802.16e communication system, the Power Saving Class Type 1 is used to maintain or deactivate the sleep mode when the MS receives a MAC SDU from the BS in the listening window according to the TTWF. Conventionally, however, a clear definition has not been given of the time at which the MS transitions back to the sleep mode after an expiration of the listening window when it intends to maintain the sleep mode. The TTWF is initially set in the MOB_SLP-REQ message and the MOB_SLP-RSP message transmitted/received by the MS and the BS. In some cases, however, a value of the TTWF should be changed during the sleep mode operation. In this case, a concrete operation is needed to change the value of the TTWF.
Also, when an MS and a BS simultaneously transmit their sleep mode requests to each other, i.e., when the MS transmits a MOB_SLP-REQ message and at the same time, the BS transmits a MOB_SLP-RSP message in an unsolicited manner before recognizing the MOB_SLP-REQ message, the MS may not determine whether the MOB_SLP-RSP message received from the BS is a response to its MOB_SLP-REQ message or has been transmitted in an unsolicited manner, which may cause a disparity of sleep mode-related parameters between the MS and the BS, leading to a malfunction of the sleep mode. Since the conventional IEEE standard 802.16e sleep mode operation does not consider the disparity, the MS may misunderstand that the BS has approved its request, based on only a ‘Sleep_Approved’ field that is included in the MOB_SLP-RSP message and indicates the approval/non-approval of activation or deactivation of the power saving class. To prevent this, the MS should verify all parameters included in the MOB_SLP-RSP message one by one to determine whether the parameters are identical to its request values.
Further, when it is negotiated between the MS and the BS to indicate a presence/absence of DL traffic using a MOB_TRF-IND message, i.e., when a ‘TRF-IND_required’ field is set to 1, which is included in the MOB_SLP-REQ/RSP message and indicates that the BS transmits at least one MOB_TRF-IND message to the MS during every listening window in the Power Saving Class Type 1, the BS should transmit the MOB_TRF-IND message during the listening window of the MS. Therefore, the BS transmits the MOB_TRF-IND message even when it has no DL traffic to transmit to the MSs that will be awake during the listening window, thereby causing a waste of resources. In addition, when the BS does not transmit the MOB_TRF-IND message, the MS may not determine whether the BS did not transmit the MOB_TRF-IND message, or the MS has failed to receive the MOB_TRF-IND message even though the BS transmitted the MOB_TRF-IND message. In this case, the MS is awakened from the sleep mode and determines that it has failed to receive the MOB_TRF-IND message.
Therefore, a need exists for a method and apparatus for controlling a sleep mode operation of a mobile station to reduce power consumption in a communication system.